The present invention relates to a chiseling, e.g., only chiseling or lathing, power tool having a pneumatic striking mechanism. In the case of a motor-driven pneumatic striking mechanism, an impacting piston is accelerated periodically in a guide tube by a pneumatic spring and strikes a point of impact on a tool or an intermediate striker. The principle of the pneumatic spring requires an airtight guidance of the impacting piston in the guide tube. Due to the high mechanical stress from repercussions or thermo-mechanical stress, the striking mechanism is heated to over 100 degrees Celsius. Furthermore, the impacting piston must move smoothly in the guide tube so it does not suppress the relatively weak coupling of the motive drive via the pneumatic spring.
The hand-held power tool according to the invention has a pneumatic striking mechanism. The striking mechanism contains a motor-driven exciter, a guide tube and a striker. A pneumatic tube configured inside the guide tube between the exciter and the striker is circumferentially sealed pressure-tight by an inner surface of the guide tube. The exciter and/or the striker are configured as a piston guided by the inner surface. For example, the exciter and striker may be configured as pistons, the exciter as a piston and the striker connected rigidly in a pot-shaped manner to the guide tube, or the striker as a piston and the exciter connected rigidly to the guide tube. An inner surface of the guide tube is provided with a plurality of molded depressions.
The shape of the inner surface is adapted to the outer contour of the piston so that the piston is cleanly guided and sealed pressure-tight. A slight deviation using depressions proves to be advantageous with regard to friction without impairing guidance or the pressure-tight seal that is essential for the concept of the pneumatic striking mechanism in particular.
One embodiment provides that the depressions have a depth between 10 μm and 100 μm. Smaller depressions show no effect; it is presumed that the lubricating film counterbalances these depressions. Larger depressions increase the abrasion of the sealing elements and therefore reduce the service life. One dimension of the depression parallel to the working axis is preferably less than 200 μm. The depressions have a surface area between 1% and 10% of the inner surface.
An inner surface of the guide tube may be provided with a plurality of grooves running oblique to the working axis and/or circular, square or diamond-shaped dents. The grooves have a length that is greater than the width by at least one order of magnitude. The dents are approximately as long as they are wide. An inner surface of the guide tube is provided with a plurality of grooves running obliquely to the working axis.
The grooves running obliquely reduce the frictional force without, as it was initially feared, considerably impairing a seal of the pneumatic chamber for the pneumatic spring. The efficiency of the striking mechanism was able to be increased overall, i.e., it was possible to increase the ratio of the impact energy emitted by the striker on the tool to the energy supplied by the motor via the exciter.
One embodiment provides that the grooves cross each other in a diamond shape. One group of grooves has a different circumferential direction around the working axis than the grooves of another group.
An angle of inclination of the grooves with respect to the working axis is preferably in a range between 30 degrees and 50 degrees. This is the case especially if the inclination of the grooves is constant over the entire length thereof. In the case of one embodiment it is provided that the angle of inclination decreases along an impact direction. The angle of inclination in the region of the point of impact is 20 degrees to 40 degrees.
The grooves extend advantageously over at least the path covered by the striker during percussive operation.
One embodiment provides that grooves of a group having a common direction of rotation run parallel to one another.
One embodiment provides that the guide tube be bent from a sheet-metal strip. Sheet-metal strips are produced by rolling methods. In doing so, the surface typically becomes very smooth and even. An embossing or scoring of the grooves proves to be especially advantageous for guide tubes formed from sheet metal. The sheet-metal strip may be connected at the two opposing edges thereof by a continuous form-fit or a seam in order to form a pressure-tight tube. The pressure-tight form-fit is guaranteed by an intermeshing.
The following description explains the invention on the basis of exemplary embodiments and Figures.